Method and device for pressure die casting

ABSTRACT

A method and apparatus for die casting comprising a stationary lower machine table (5) provided with a stationary lower mold part (24) which cooperates with a vertically moveable upper mold part (22) mounted on an upper machine table (10) and displaceable by hydraulic cylinders (7). The upper machine table (10) and the upper mold part are provided with a filling chamber (20) for receiving a measured quantity of a molten metal which is to be pressed into the mold by an upper piston (18). A lower piston (43) is moved to a variable position relative to the filling chamber, prior to the molten metal filling, and thereafter the upper piston (18) is moved into contact with a top surface of molten metal, without any air-entrapment or with only a minor entrapment of air. After this has occurred, the upper piston and the lower piston are displaced jointly downward until the lower piston is in a casting position in the lower mold part, whereupon the upper piston is displaced further downward to feed the molten metal and maintain pressure of the molten metal until the casting operation is completed. The cylinder casing (38) of the lower piston is rigidly connected to a mechanism (29-37) designed to eject the casting produced.

The present invention relates to a method of die casting of the type setforth in more detail below. Also, the invention relates to a device usedin die casting according to the first claim, which relates to a device.

In order to rationally utilize the capacity of a die casting machine itis often used to cast different objects. In order to cast such objects,i.e. in order to pass from the casting of a certain object type toanother such object type, the lower as well as the upper piston and alsothe tube-shaped filling chamber usually are exchanged and of course alsothe mold and the tube, which surrounds the lower piston. Of course, thisis always a time-consuming and expensive procedure, which is somethingobjectionable. Also, the casting procedure is obstructed in this way,since e.g. constantly altered diameters of casting pistons, used at themoment, result in fluctuating pressures, which usually are notacceptable or even attainable. Consequently, an alteration of thecasting piston diameter, i.e. a casting piston exchange, has to beaccompanyed by a corresponding alteration of the pressing pressure ofthe machine. This leads automatically to a great risk, that personnelwill make miscalculations or misadjustments as regards the pressure, aquality deterioration or even a rejection of castings producedresulting. Also, undesirable air entrapments can not always bepositively avoided in the known die casting machines.

The object of the present invention is to improve and develop the knowndie casting methods and devices in the above-mentioned respects. Also,another object is to contribute to the advancement of the art in thisfield in various respects and create opportunities for a rationaltechnique and products having a satisfactory and uniform quality.

These objects are attained by the method and apparatus more clearly setforth in the following description and appended drawings.

Additional characterizing features and advantages of the presentinvention are set forth in the following description, reference beingmade to the accompanying drawings, which partially schematicly show anembodiment, which is not limiting, of a die casting device according tothe invention, and in which:

FIG. 1 shows a device according to the invention in its castingposition, i.e. after a closing of the molds and a filling of liquidmetal but before the casting itself; FIG. 2 shows the device accordingto FIG. 1 after the completed casting; and

FIG. 3 shows the device according to FIG. 1 after the discharge of thecastings produced, being prepared to return to the starting position forthe next casting cycle.

In the drawings a device 1 according to the invention, shown in itsentirety, comprises a machine frame 2, which mainly comprises an upperbase plate 3, from which four machine bearers 4 project downwards, thelower ends of which support a lower machine table 5, which is supportedby feet 6.

Upper base plate 3 supports four hydraulic form cylinders 7, designed toopen and close a mold, which is described in the following text, bymeans of a control system, known per se (not shown). Piston rods 9project from form cylinders 7 downwards through bores 8 in the baseplate, and an upper machine table 10 is suspended from the lower ends ofsaid piston rods and is displaceable along machine bearers 4 by means ofguide bushings 11.

A yoke 13 is positioned above upper machine table 10 by means of spacingrods 12, at a distance from and preferably centered above this machinetable. Yoke 13 supports an upper piston cylinder 14, which projectsupwards through an opening 15 in the base plate, which opening is wideenough to allow also the yoke in its position according to FIG. 3 topass.

A piston rod 17 projects downwards through an opening 16 in yoke 13 andis provided with an upper piston 18, which is inserted into a tube 19,which is fastened to upper machine table 10, encloses a filling chamber20 and is provided with a filling funnel 21, designed to introduce acertain measured amount of liquid metal.

An upper mold part 22 is fastened to the underside of the upper machinetable, and its underside is in its turn provided with an upper cavity23.

A lower form part 24 is supported on machine table 5 and its upside isprovided, in an analogous manner, with a lower cavity 25.

A holding plate 27 is mounted below machine table 5 through spacing rods26, at a distance from the same, for ejector cylinders 28, mounted belowthe holding plate. Their piston rods 29 extend upwards through openings30 in holding plate 27 to jointly support a bridge 31, which is mountedto be displaceable between machine table 5 and holding plate 27. Bridge31 supports in turn 2-12 ejector rods 32, which extend upwards throughbores 33 in machine table 5 in order to, above the latter, support acommon connecting plate 34 having upper ejector plates 35 and ejectorpins 37, which are inserted from said ejector plates into lower moldpart 24 in control holes 36 in the latter, by means of which ejectorpins the castings, after the completed casting operation, can beejected, when the mold has been opened up.

A lower piston cylinder 38 is mounted below bridge 31 in a centralposition, in which cylinder a piston 39 is mounted, from which a pistonrod 40 issues, which extends through openings 41 and 42 respectively inbridge 31 and machine table 5 respectively, in order to, in its freeends support a lower piston 43, which is guided in a bushing 44 in thelower mold part and in bushing 45, designed as a continuation of bushing44, in the upper mold part. Also, tube 19 and bushing 45 can be designedas a unit. Also, said bushings can be cooled directly or indirectly in away known per se, not shown in great detail in this context. However,cooling ducts are jointly indicated through reference numeral 46 in FIG.1.

Lower piston 43 and piston rod 40 suitably are cooled, too, and in thatcase a connection 47 from piston 39 to a cooling equipment, not shown,can extend downwards through cylinder 38.

The device, described in this way, can be used as follows: FIG. 1 shows,as has been mentioned, the device in its casting position, the moldsbeing closed, e.g. with an enclosed mold insert 48. A certain measuredamount of liquid metal, preferably being a non-iron-alloy, has alreadybeen filled into chamber 20 and is designated 49. The surface of thefilled metal suitably reaches the lower edge of funnel 21 or possibly asmall distance below the same, provided a very small required amount ofair is to be enclosed between the upper piston and the metal during thefollowing casting operation. In order to attain this precise adjustmentone has to pay attention to the measured amount of metal as well as, andthis is most important, the position of the lower piston, whichaccording to the invention is adjustable in a required elevationposition, in order to allow an exact calculatable filling of metal, andan optimal carrying out of the casting operation, with the leastpossible excess amount. The position of the lower piston is adjusted andindicated in a suitable way, e.g. through a programmable electronicallyoperated display panel 60.

The upper piston is then lowered to immediately above the metal surface,in which position the device is programmed in such a way, that the upperpiston and the lower piston start a joint movement downwards, until thelower piston has reached a position according to FIG. 2, in which thelower piston stops, i.e. immediately below feeding ducts 1 between themold parts. The speeds of the pistons during the movement now describedpreferably are the same. When the lower piston has stopped, the upperpiston moves further downwards to a position according to FIG. 2 inorder to bring forward all of the filled amount of metal and press thesame into the entire mold, a certain excess amount, a so called pressingbriquette 50, remaining in the lowest portion of the filling chamber.

When the metal has solidified in the mold, after e.g. 2-20 seconds, formcylinders 7 are activated and piston rods 9 will then pull upper machinetable 10 upwards with upper mold part 22 and the upside of the castingwill be exposed. In order to completely remove briquette 50 from fillingchamber 20 upper piston cylinder 14 is kept active and consequently, itwill push the piston rod outwards successively, when machine table 5 ismoved upwards, the upper piston all the time being pressed against thebriquette, until the upper machine table has reached its upper startingposition according to FIG. 3, in which the upper piston cylinder isactivated to pull the piston into the position shown in FIG. 3. Ofcourse, piston rod 17 can be pulled inwards somewhat earlier, e.g. assoon as a relative position has been reached, in which the briquette hasleft the filling chamber.

Cylinders 28 are now activated and bridge 31 is lifted and finallyejector pins 37 are activated as a result, the castings produced beingcompletely set free, also from the lower mold part. The lower pistoncontributes to this also, since its cylinder is fastened to the bridge,which results in that also the lower piston besides the ejector pins aredisplaced in relation to the lower mold part. When cylinders 28 havereturned to their starting position according to FIG. 1, cylinder 38 isactivated and its piston 39 lifts the lower piston to a positionaccording to FIG. 3, which is the same as the position shown in FIG. 1,and in this way a new casting cycle can be initiated.

The free end surface of the lower piston and preferably also itsenvelope surface are, according to a preferred embodiment of theinvention, provided with a ceramic coating in order to stop a wearand/or provide a heat insulator to a limited degree. The wall of thefilling chamber can also be made of a ceramic material. In this way themetal will be cooled to a minimum in the filling chamber, which isdesirable, while the thermal expansion of tube 19 and the lower pistonrespectively also is comparatively limited, which is desirable in orderto maintain the required limits. Also, the upper piston can be cooled inorder to attain a faster solidification, and its envelope surface can,in order to prevent a wear, be provided with a ceramic coating.

In order to obtain the best possible casting quality the presentinvention, with all its characterizing features, strives to achieve aconsistent control of the solidification from the outside and inwards,e.g. briquette 50 in this connection being kept as hot as possible atits underside and at the lateral sides, while a cooling and then also asolidification from the upside is facilitated. The underside or the freeend surface of the upper piston can also be made of a material having asatisfactory thermal expansion coefficient, e.g. molybdenum alloys andparticularly such an alloy with the brand name of TZM.

While the transition period from one casting type to another is 3-4 h ina conventional casting machine, we have found that the correspondingtransition period in a device or machine according to the presentinvention can be as short as 10-15 minutes. Thus, important timesavingsand method improvement gains are made possible in accordance with thepresent invention.

I claim:
 1. A method of die casting an article in an apparatus having alower piston (43) guided by a stationary lower mold part (24), a uppermachine table (10) with an upper mold part (22), cooperating with thelower mold part (24), mounted on a bottom surface thereof moveablevertically relative to the lower mold part (24), a filling chamber (20)carried by the upper mold part for containing a desired quantity ofmolten metal (49), the lower piston (43), when engaged with the fillingchamber, forming a base portion of the filling chamber for temporarilysupporting the molten metal, said method comprising the steps of:movingthe lower piston (43) relative to the filling chamber to a variableinitial starting position prior to supplying the molten metal to thefilling chamber, the variable initial starting position of the lowerpiston (43) being selected so that the quantity of molten metal to becontained within the filling chamber is equivalent to a contemplatedcasting plus a required excess amount, after supplying the molten metalto the filling chamber, moving an upper piston (18) into contact with atop surface of the molten metal, without any substantial air-entrapmentin the filling chamber, for pressure feeding the molten metal into themold (22, 24), displaying jointly the lower piston, the molten metal andthe upper piston vertically downward until the molten metal is incommunication with the feeding duct, stopping further downward motion ofthe lower piston, upon the molten metal communicating with the feedingduct, while continuing further downward motion of the upper piston topressure feed the molten metal into the mold parts and subject themolten metal to continued pressure until the casting operation iscompleted, and ejecting the cast article from at least one of the moldparts by an ejector mechanism (29-37).
 2. A method according to claim 1,further comprising the step of indicating the initial starting positionof the lower piston position electronically with a display panel.
 3. Amethod according to claim 1, further comprising the step of, after themolten metal has solidified in the mold (22, 24), moving the upper moldpart vertically upward away from the lower mold part to expose the castarticle and maintaining pressure on a briquette (50) which formed in thefilling chamber (20) during this vertical upward movement so that thebriquette exits the filling chamber.
 4. A method according to claim 3,further comprising the step of moving the upper piston to its startingposition for a new casting cycle after the briquette exits the fillingchamber.
 5. A method according to claim 1, further comprising the stepof moving the lower piston, after ejecting the cast article from themold, to a desired initial starting position for a new casting cycle. 6.A method according to claim 1, further comprising the step of heatinsulating at least one of the inner wall of the filling chamber (20)and at least a portion of the lower piston (43).
 7. A method accordingto claim 6, further comprising the step of using a ceramic coating forthe heat insulating.
 8. A method according to claim 1, furthercomprising the step of using a ceramic coating for heat insulating.
 9. Amethod according to claim 8, further comprising the step of using amolybdenum alloy for making at least a free end surface of the upperpiston heat conductive.
 10. A device for die casting comprising astationary lower machine table (5) having a stationary lower mold part(24) on an upper surface thereof, an upper mold part (22), cooperatingwith the lower mold part, mounted on a lower surface of an upper machinetable (10), the upper mold part and the upper machine table beingvertically movable relative to the lower mold part by hydrauliccylinders (7) connected to support means, the upper machine table andthe upper mold part jointly forming a filling chamber (20) for receivingmolten metal to be cast, a hydraulic actuated upper piston (18) attachedto the support means to facilitate casting of the molten metal into thecooperating mold parts, and a hydraulic actuated lower piston (43),guided by the lower mold part, for temporarily supporting the moltenmetal, and the lower piston (43) cooperating with the upper piston (18)to facilitate casting of the molten metal,wherein said device furtherincludes means for moving the lower piston (43) relative to the fillingchamber to a variable initial starting position prior to supplying themolten metal to the filling chamber, the variable initial startingposition of the lower piston (43) being selected so that the quantity ofmolten metal to be contained within the filling chamber is equivalent toa contemplated casting plus a required excess amount, means for movingan upper piston (18) into contact with a top surface of the moltenmetal, after supplying the molten metal to the filling chamber, withoutany substantial air-entrapment in the filling chamber for pressurefeeding the molten metal into the mold parts (22, 24), means for jointlydisplacing the lower piston, the molten metal and the upper pistonvertically downward until the molten metal is in communication with thefeeding duct, means for stopping further downward motion of the lowerpiston, upon the molten metal communicating with the feeding duct, whilecontinuing further downward motion of the upper piston to pressure feedthe molten metal into the mold parts and subject the molten metal tocontinued pressure until the casting operation is completed, means forejecting the cast article from at least one of the mold parts aftercasting, and the support means is an upper base plate (3), feet supportthe lower machine table (5) which supports a plurality of machinebearers (4) along which the upper machine table (10) is verticallydisplaceable, the plurality of machine bearers (4) support the upperbase plate (3), adjacent an end opposite the lower machine table, towhich the cylinders (7) for actuating the upper mold part (22) aremounted, and the upper machine table (10) supports, via spacing rods(12), a yoke (13) to which is mounted the upper piston cylinder (14)carrying the upper piston (18), and the yoke is vertically displaceablethrough an opening (15) in the upper base plate (3) during upwardvertical movement of the upper machine table.
 11. A device according toclaim 10, wherein the mold parts (22, 24) and at least one of the uppermachine table (10), the lower piston (43) and the upper piston (18) areprovided with a cooling mechanism (46 and 47 respectively).
 12. A deviceaccording to claim 10, wherein at least one of an inner surface of thefilling chamber (20) and a surface of the lower piston (43) comprises aceramic material which insulates and reduces wear.
 13. A deviceaccording to claim 10, wherein at least one of the filling chamber (20)and a free end surface of the lower piston (43) comprises a materialwhich heat insulates.
 14. A device according to claim 13, wherein thematerial which heat insulates comprises a ceramic material.
 15. A deviceaccording to claim 10, wherein at least a free end surface of the upperpiston (18) contains a molybdenum alloy.
 16. A device for die castingcomprising a stationary lower machine table (5) having a stationarylower mold part (24) on an upper surface thereof, an upper mold part(22), cooperating with the lower mold part, mounted on a lower surfaceof an upper machine table (10), the upper mold part and the uppermachine table being vertically movable relative to the lower mold partby hydraulic cylinders (7) connected to support means, the upper machinetable and the upper mold part jointly forming a filling chamber (20) forreceiving molten metal to be cast, a hydraulic actuated upper piston(18) attached to the support means to facilitate casting of the moltenmetal into the cooperating mold parts, and a hydraulic actuated lowerpiston (43), guided by the lower mold part, for temporarily supportingthe molten metal, and the lower piston (43) cooperating with the upperpiston (18) to facilitate casting of the molten metal,wherein saiddevice further includes means for moving the lower piston (43) relativeto the filling chamber to a variable initial starting position prior tosupplying the molten metal to the filling chamber, the variable initialstarting position of the lower piston (43) being selected so that thequantity of molten metal to be contained within the filling chamber isequivalent to a contemplated casting plus a required excess amount,means for moving an upper piston (18) into contact with a top surface ofthe molten metal, after supplying the molten metal to the fillingchamber, without any substantial air-entrapment in the filling chamberfor pressure feeding the molten metal into the mold parts (22, 24),means for jointly displacing the lower piston, the molten metal and theupper piston vertically downward until the molten metal is incommunication with a feeding duct, means for stopping further downwardmotion of the lower piston, upon the molten metal communicating with thefeeding duct, while continuing further downward motion of the upperpiston to pressure feed the molten metal into the mold parts and subjectthe molten metal to continued pressure until the casting operation iscompleted, means for ejecting the cast article from at least one of themold parts after casting, and a holding plate (27) is mounted to a lowersurface of the lower machine table (5) and spaced therefrom at a desireddistance by spacing rods (26), a plurality of ejector cylinders (28) aremounted on a lower surface of the holding plate and are connected, vialower ejector rods (29), to a bridge (31) mounted between the lowermachine table and the holding plate for vertically displacing the bridge(31), a lower piston cylinder (38) and upper ejector rods (32) aremounted on the bridge, the upper ejector rods (32) extend through anopening in the lower machine table and support a common connection plate(34), and at least one ejector plate (35), connected to the connectionplate (34), supports ejector pins (37) guided into the lower mold part(24) for ejecting the cast article.
 17. A device according to claim 16,wherein at least one of an inner surface of the filling chamber (20) anda surface of the lower piston (43) comprises a ceramic material whichinsulates and reduces wear.
 18. A device according to claim 16, whereinat least one of the filling chamber (20) and a free end surface of thelower piston (43) comprises a material which heat insulates.
 19. Adevice according to claim 18, wherein the material which heat insulatescomprises a ceramic material.
 20. A device according to claim 16,wherein at least a free end surface of the upper piston (18) contains amolybdenum alloy.